NIA Pilot Research Program, Topic 14: Sensory and Motor Processing The endurance capacity of a muscle will vary with the task performed. However, the influence of age-related changes within the neuromuscular system on the task-dependent nature of muscle fatigue is unexplored in older adults. This raises the following question: Does age alter the ability of the neuromuscular system to adapt to different fatiguing tasks? In young adults, the endurance time when performing a submaximal isometric contraction was longer when maintaining a force and pushing against a force transducer (force task) compared with maintaining a constant elbow angle while supporting an equivalent inertial load at the wrist (position task) (Hunter et al. 2002). The briefer endurance time for the position task was attributable to neural mechanisms including a greater rate of descending drive. Given that age-related changes in the neuromuscular system will predispose the older adult to impairment within the nervous system during a fatiguing task, this RO3 application examines the following hypothesis: the difference in endurance time of a submaximal fatiguing contraction for a position task compared with a force task, will be greater in older adults compared with young adults due to age-related impairment in neural mechanisms. There are three specific aims to test this hypothesis. The first aim will compare the endurance time and muscle activation patterns of the elbow flexor muscles in young and older adults for a position task and force task. Aims 2 and 3 will evaluate potential mechanisms for the age-related differences in muscle fatigue of the two tasks, by measuring indices of descending drive and peripheral afferent feedback from the muscle during the fatiguing contractions. These measures include muscle sympathetic nerve activity, heart rate, fluctuations in motor Output, sense of effort and the pressor response. This application proposes a novel approach to examine muscle fatigue in older adults. The findings will provide preliminary information on the type of load and support preferred by older adults for long-duration submaximal tasks during daily tasks. This will constitute new information on the task-dependent nature of muscle fatigue in older adults. These preliminary findings will form the foundation for an RO 1 application that will further explore the mechanisms and task dependency of muscle fatigue in older adults.